Lithium-ion and lithium (Li) metal cells (including lithium metal secondary cell, lithium-sulfur cell, lithium-selenium cell, Li-air cell, etc.) are considered promising power sources for electric vehicle (EV), hybrid electric vehicle (HEV), and portable electronic devices, such as lap-top computers and mobile phones. Lithium metal has the highest capacity (3,861 mAh/g) compared to any other metal or metal-intercalated compound (except Li4.4Si) as an anode active material. Hence, in general, rechargeable Li metal batteries have a significantly higher energy density than lithium ion batteries.
Historically, rechargeable lithium metal batteries were produced using non-lithiated compounds having high specific capacities, such as TiS2, MoS2, MnO2, CoO2 and V2O5, as the cathode active materials, which were coupled with a lithium metal anode. When the battery was discharged, lithium ions were dissolved from the lithium metal anode and transferred to the cathode through the electrolyte and, thus, the cathode became lithiated. Unfortunately, upon cycling, the lithium metal resulted in the formation of dendrites that ultimately caused unsafe conditions in the battery. As a result, the production of these types of secondary batteries was stopped in the early 1990's giving ways to lithium-ion batteries.
Even now, cycling stability and safety concerns remain the primary factors preventing the further commercialization of Li metal batteries for EV, HEV, and microelectronic device applications. These issues are primarily due to the high tendency for Li to form dendrite structures during repeated charge-discharge cycles or an overcharge, leading to internal electrical shorting and thermal runaway. Many attempts have been made to address the dendrite-related issues, as briefly summarized below:
Fauteux, et al. [D. Fauteux, et al., “Secondary Electrolytic Cell and Electrolytic Process,” U.S. Pat. No. 5,434,021, Jul. 18, 1995] applied to a metal anode a protective surface layer (e.g., a mixture of polynuclear aromatic and polyethylene oxide) that enables transfer of metal ions from the metal anode to the electrolyte and back. The surface layer is also electronically conductive so that the ions will be uniformly attracted back onto the metal anode during electrodeposition (i.e. during battery recharge). Alamgir, et al. [M. Alamgir, et al. “Solid polymer electrolyte batteries containing metallocenes,” U.S. Pat. No. 5,536,599, Jul. 16, 1996] used ferrocenes to prevent chemical overcharge and dendrite formation in a solid polymer electrolyte-based rechargeable battery.
Skotheim [T. A. Skotheim, “Stabilized Anode for Lithium-Polymer Battery,” U.S. Pat. No. 5,648,187 (Jul. 15, 1997); U.S. Pat. No. 5,961,672 (Oct. 5, 1999)] provided a Li metal anode that was stabilized against the dendrite formation by the use of a vacuum-evaporated thin film of a Li ion-conducting polymer interposed between the Li metal anode and the electrolyte. Skotheim, et al. [T. A. Skotheim, et al. “Lithium Anodes for Electrochemical Cells,” U.S. Pat. No. 6,733,924 (May 11, 2004); U.S. Pat. No. 6,797,428 (Sep. 28, 2004); U.S. Pat. No. 6,936,381 (Aug. 30, 2005); and U.S. Pat. No. 7,247,408 (Jul. 24, 2007)] further proposed a multilayer anode structure consisting of a Li metal-based first layer, a second layer of a temporary protective metal (e.g., Cu, Mg, and Al), and a third layer that is composed of at least one layer (typically 2 or more layers) of a single ion-conducting glass, such as lithium silicate and lithium phosphate, or polymer. It is clear that such an anode structure, consisting of at least 3 or 4 layers, is too complex and too costly to make and use.
Protective coatings for Li anodes, such as glassy surface layers of LiI—Li3PO4—P2S5, may be obtained from plasma assisted deposition [S. J. Visco, et al., “Protective Coatings for Negative Electrodes,” U.S. Pat. No. 6,025,094 (Feb. 15, 2000)]. Complex, multi-layer protective coatings were also proposed by Visco, et al. [S. J. Visco, et al., “Protected Active Metal Electrode and Battery Cell Structures with Non-aqueous Interlayer Architecture,” U.S. Pat. No. 7,282,295 (Oct. 16, 2007); U.S. Pat. No. 7,282,296 (Oct. 16, 2007); and U.S. Pat. No. 7,282,302 (Oct. 16, 2007)].
Despite these earlier efforts, no rechargeable Li metal batteries have yet succeeded in the market place. This is likely due to the notion that these prior art approaches still have major deficiencies. For instance, in several cases, the anode or electrolyte structures are too complex. In others, the materials are too costly or the processes for making these materials are too laborious or difficult. Solid electrolytes typically have a low lithium ion conductivity, are difficult to produce and difficult to implement into a battery.
Furthermore, solid electrolyte, as the sole electrolyte in a cell or as an anode-protecting layer (interposed between the lithium film and the liquid electrolyte) does not have and cannot maintain a good contact with the lithium metal. This effectively reduces the effectiveness of the electrolyte to support dissolution of lithium ions (during battery discharge), transport lithium ions, and allowing the lithium ions to re-deposit back onto the lithium anode (during battery recharge).
Another major issue associated with the lithium metal anode is the continuing reactions between electrolyte and lithium metal, leading to repeated formation of “dead lithium-containing species” that cannot be re-deposited back to the anode and become isolated from the anode. These reactions continue to irreversibly consume electrolyte and lithium metal, resulting in rapid capacity decay. In order to compensate for this continuing loss of lithium metal, an excessive amount of lithium metal (3-5 times higher amount than what would be required) is typically implemented at the anode when the battery is made. This adds not only costs but also a significant weight and volume to a battery, reducing the energy density of the battery cell. This important issue has been largely ignored and there has been no plausible solution to this problem in battery industry.
Clearly, an urgent need exists for a simpler, more cost-effective, and easier-to-implement approach to preventing Li metal dendrite-induced internal short circuit and thermal runaway problems in Li metal batteries, and to reducing or eliminating the detrimental reactions between lithium metal and the electrolyte.
Hence, an object of the present invention was to provide an effective way to overcome the lithium metal dendrite and reaction problems in all types of Li metal batteries having a lithium metal anode. A specific object of the present invention was to provide a lithium metal cell that exhibits a high specific capacity, high specific energy, high degree of safety, and a long and stable cycle life.